1. Field of the Invention
This invention relates to protective devices for starters (hereinafter referred to as "starter protective devices", when applicable), and more particularly to a starter protective device which automatically stops the starter after the start of the engine.
2. Prior Art
A conventional starter protective device of this type is as shown in FIG. 3. In FIG. 3, reference numeral 1 designates an engine; 2, a starter for starting the engine 1; 3, a battery; 4, a key switch on a vehicle; 5, a decision unit made up of electronic circuits; 6, a sensor for detecting the number of revolutions (speed) of the engine to detect the operating condition of the engine; 7, an electro-magnetic switch which is connected between the starter operating contact 4a of the key switch and a current supplying line 8 connected to the starter 2; and 9, a transistor for controlling the on-off operation of the exciting coil 7a of the electro-magnetic switch in response to the output of the decision unit 5.
When, in the device thus organized, the key switch 4 is positioned to the contact 4b, the decision unit 5 is energized, so that the transistor 9 is rendered conductive (on). Under this condition, the key switch 4 is positioned to the starter operating contact 4a, so that current flows from the battery 3 to the exciting coil 7a. As a result, the movable contact 7b of the electromagnetic switch 7 is brought into contact with the stationary contacts 7c and 7d, whereby the starter 2 is energized, and the engine 1 is started.
When it is detected by the sensor 6 that the number of revolutions of the engine reaches a predetermined value Ne (FIG. 4), the decision unit 5 operates to render the transistor 9 non-conductive (off), so that the stationary contacts of the electro-magnetic switch 7 are opened; that is, the operation of the starter is suspended.
As was described above, in order to prevent the long time operation of the starter, or its over-run attributing to the engine, with the conventional starter protective device, the operation of the starter is stopped when the decision unit 5 determines that the number of revolution of the engine 1 has exceeded the predetermined value Ne which is the start decision reference. However, the engine may suffer from the following difficulty when operated at cold districts: As shown in FIG. 4, even if the number of revolutions of the engine exceeds the value Ne momentarily, it is decreased as indicated by the dotted line C, so that the engine is stopped. Therefore, in the conventional starter protective device, the time interval (or time lag) T.sub.L which elapses from the time instant A that the number of revolutions exceed the value Ne until the time instant B occurs is employed as another start decision reference, and it is determined whether or not the number of revolutions of the engine exceeds the value Ne over the time interval T.sub.L or at the time instants A and B.
However, the conventional starter protective device is still insufficient in performance; that is, it is impossible for it to determine it positively at all times whether or not the engine has been started. Therefore, with the device, the starter may be stopped before the engine is started. This is because starting an engine depends greatly on the ambient temperature or the conditions of the engine. Thus, it is not suitable that the number of revolutions Ne and the time lag T.sub.L which are constant are employed as the start decision references. This difficulty may be overcome by increasing the number of revolutions Ne and the lag time T.sub.L ; however, it is not practical to merely do so, because if those data are increased, then in the case when the engine is smoothly started, the starter is energized for a long period of time, or its over-run is caused, with the result that the service life of the starter is decreased as much; that is, the protective device does not work